A capacitive humidity sensor and a method for manufacturing a capacitive humidity sensor are disclosed in, for example, JP-A-243690, which corresponds to U.S. Pat. No. 6,580,600-B2 and US Patent Application Publication No. 2002/0114125-A1. The sensor includes a humidity sensing element, a capacitance of which changes with humidity. The sensor 90 is shown in FIG. 4. The sensor 90 includes a humidity sensitive portion 30 and a circuit portion 40, which are disposed on one side of a semiconductor substrate 1.
The humidity sensitive portion 30 is composed of a pair of electrodes 91a, 91b, a silicon oxide film 2, a silicon nitride film 3 and a humidity sensitive film 4. The silicon oxide film 2 is formed on the substrate 1. The electrodes 91a, 91b face each other, are disposed on the silicon oxide film 2, and are separated each other. The silicon nitride film 3 and the humidity sensitive film 4 are formed to cover the electrodes 91a, 91b. The humidity sensitive film 4 is made of material, dielectric constant of which changes in accordance with humidity change of atmosphere. Accordingly, a capacitance between the electrodes 91a, 91b in the humidity sensitive portion 30 changes in accordance with the humidity change of the atmosphere around the sensor 90.
The circuit portion 40 controls and drives, i.e., energizes the humidity sensing element 10 in the humidity sensitive portion 30. The circuit portion 40 includes a reference capacitance portion 42 and a CMOS transistor portion 41. The reference capacitance portion 42 provides a reference capacitance, and the CMOS transistor portion 41 includes a CMOS transistor and the like. The capacitance change between the electrodes 91a, 91b in the humidity sensitive portion 30 is compared with the reference capacitance of the reference capacitance portion 42, so that a result of the comparison is processed in the CMOS transistor portion 41. Thus, the sensor 90 measures the capacitance change between the electrodes 91a, 91b in accordance with the humidity change so that the humidity of the atmosphere is detected.
The humidity sensitive film 4 in the above sensor 90 is manufactured by a screen printing method. Thus, the manufacturing cost of the sensor 90 becomes low. However, the humidity sensitive film 4 formed by the screen printing method has following problems.
The material for providing the humidity sensitive film 4 is firstly mixed into a paste. Then, the paste is printed by the screen printing method. However, the humidity sensitive film 4 formed by the screen printing method has variation of viscosity in each part and variation of printing pressure in each part. Thus, saddle phenomenon may be occurred, as shown in FIG. 6. The saddle phenomenon is such that a thickness of a center of the humidity sensitive film 4 is different from a thickness of a periphery of the humidity sensitive film 4 after the humidity sensitive film 4 is printed. When the saddle phenomenon is occurred in the humidity sensitive film 4 in the sensing element 10, the characteristics of the sensor 90 may be varied. Thus, detection accuracy of the humidity is reduced.